Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults, accounting for most of 18,000 primary brain tumor cases each year in the US. Prognosis is dismal with a median survival of 12-15 mo, despite use of multimodality treatment. Novel therapeutic agents are urgently needed. During the current funding period of the Mayo Brain SPORE, our group was the first to demonstrate that engineered measles virus (MV) strains have significant antitumor activity against gliomas. Their tumor specificity is due to abundant expression of the MV receptor CD46 in glioma cells. The virus upon entry in the tumor cells, causes membrane fusion with neighboring cells, syncytia formation and death. In addition, we have translated this approach into the first human clinical trial of a measles virus derivative producing human carcinoembryonic antigen, MV-CEA (CEA added to facilitate viral monitoring) in recurrent GBM patients. We now hypothesize that by increasing the efficiency and extent of tumor cell destruction and by introducing a therapeutic transgene, we can further augment the antitumor activity of measles virotherapy in gliomas. We propose to accomplish this by testing the translational potential of three novel approaches; a different measles virus strain, MV-NIS, which encodes the sodium iodine symporter (NIS) gene, thus allowing Imaging of viral distribution in vivo; enhancing MV-NIS oncolysis, by exploiting NIS as therapeutic transgene with application of the beta and gamma emitter (radiovirotherapy); and combining measles virus derivatives with cyclophosphamide, an agent that has been shown to suppress anti-viral innate and adaptive immunity, and increase viral proliferation in tumors. This grant proposal has therefore the following specific aims: 1) to evaluate the therapeutic potential of MV-NIS based virotherapy and radiovirotherapy against GBM and compare its antitumor activity with MV-CEA; 2) to test combinatorial strategies with cyclophosphamide, a suppressant of the innate immune response, in order to further increase the potency of measles virotherapy or radiovirotherapy; 3) to perform toxicology and biodistribution studies in measles susceptible transgenic mice and Rhesus macaques in order to determine the safety of the optimal therapeutic strategy to be tested in a follow-up clinical trial; 4) to employ the approach with the optimal safety/efficacy profile in a subsequent phase I clinical trial in patients with recurrent glioblastoma multiforme.